The present invention relates to improvements of a rolling bearing for use in a precision equipment, a food machine, and a semiconductor-related equipment, and improvements of a linear guide apparatus, and a ball screw (hereinafter called generally a rolling device). In particular, the present invention relates to improvements of material compositions of component parts to enhance the function of the rolling device.
In a precision equipment, a food machine, a semiconductor-related equipment and the like, various kinds of rolling devices such as a rolling bearing, a linear guide (linear guide apparatus), and a ball screw are employed conventionally. Each of these rolling devices includes component parts of an outer member, an inner member and rolling elements, while the rolling elements are structured such that they can be rolled not only with respect to a first contact surface, which is the contact surface of the outer member with the rolling elements, but also with respect to a second contact surface which is the contact surface of the inner member with the rolling elements. In more particular, the term "the outer member of the rolling device" is used here to indicate an outer race in a rolling bearing, a slider or a guide rail in a linear guide, and a nut in a ball screw. Also, the inner member of the rolling device is used here to indicate an inner race in a rolling bearing, a guide rail or a slider in a linear guide, and a screw shaft in a ball screw.
Therefore, referring to the first contact surface which is the contact surface of the outer member with the rolling elements and the second contact surface which is the contact surface of the inner member with the rolling elements, in the case of the rolling bearing, the raceway surface of the outer race is the first contact surface and the raceway surface of the inner race is the second surface; also, in the case of the linear guide, the raceway groove of the slider (or guide rail) is the first contact surface and the raceway groove of the guide rail (or slider) is the second contact surface; and, further, in the case of the ball screw, the screw groove of the nut is the first contact surface and the screw groove of the screw shaft is the second contact surface.
As for material used to form not only balls and various rollers serving as the rolling elements of the rolling bearing or other rolling devices but also the inner and outer races, slider, nut, and screw shaft serving as the inner and outer members of the rolling bearing and rolling device, SUJ2 is generally employed for bearing steel. Also steel material corresponding to SCR420 is employed for case hardening steel. Since the rolling bearing or other rolling device is used under high contact pressure with repetitive shearing stresses applied thereto, the bearing steel is hardened and tempered, while the case hardening steel is carburized or carbonitrided and is then hardened and tempered in order to stand such shearing stresses and secure a necessary rolling fatigue strength. Hence, the bearing steel and case hardening steel can have the hardness in the range of H.sub.R C 58-64. However, the rolling bearing or other rolling device is used under various environments. For example, when the rolling bearing formed of SUJ2 or the steel material corresponding to SCR420 is used under a corrosive environment such as a water or seawater mixed environment, a wet environment, or other similar environments, the rolling bearing gathers rust and thus cannot be used in an early stage. In view of this, especially, in the rolling bearing and rolling device for use in a precision equipment, a food machine and the like in which the rust gathering must be avoided, there has been conventionally used martensitic steel SUS440 or the like as high chromium stainless bearing steel which is excellent in corrosion resistance and has the hardness of H.sub.R C 58 or more necessary for the bearing.
Under the corrosive environments such as the water or seawater mixed environment, the wet environment and the like, generally, relatively neutral water having pH 5-9 adheres to the rolling device. Also, in special cases, the rolling device is used in a special solution such as a weak acid solution, a halide solution or the like, or in steam. Especially, in the case of a reducing acid such as a sulfuric acid, a hydrochloric acid or the like, even if several % of such acid is contained in water, such acid can attack and corrode heavily the passivated film (film oxide) of the stainless steel so that the stainless steel is forced to reach its corrosion life. In such case, a surface treatment such as a hard Cr plating treatment, a fluoride plating treatment or other similar treatment must be given to the rolling device before it can be practically used.
However, in the high chromium stainless steel, when the contents of C and Cr are higher, a problem arises. For example, when it contains 0.6 wt % of C or more, due to combination of such carbon with a large amount of chromium, there are produced a large number of eutectic carbides each having a large size of 10 .mu.m or more, which not only reduces the fatigue strength, toughness, corrosion resistance, workability and the like of the high chromium stainless steel but also deteriorates the forgiability, machinability and the like of the high chromium stainless steel.
Also, the existence of the large-size eutectic carbides raises another problem that it has an undesirable influence on the acoustic characteristic of the rolling bearing and rolling device. The term "acoustic characteristic" expresses the degree of the low levels of the noise that is produced by vibrations generated due to operation of the rolling bearing or other rolling device. This does not matter so much in a machine tool, a construction machine, and the like. However, in a relatively small-size stainless steel ball bearing for use in a precision equipment such as HDD, VTR or the like which detests vibrations bitterly, the deteriorated acoustic characteristic provides a significant problem. That is, the vibrations generated in the rolling bearing or other rolling device depend greatly on the dimensional precision of the inner and outer members and rolling elements. For this reason, when the rolling bearing or other rolling device is formed of such material as contains the large-size eutectic carbides, the large-size eutectic carbides provide a factor to impede the accomplishment of the precision in finishing component parts of the rolling device. Also, during use of the rolling device, a wear difference between the base material of the rolling device and the eutectic carbides is produced to thereby cause the precision of roughness and the like of the rolling device to be lowered, that is, to provide a precision reducing factor, which results in the increased noise. Here, such lowered acoustic characteristic can be caused not only by the above-mentioned large-size eutectic carbides but also by the amount of retained austenite.
Such large-size eutectic carbides not only lower the acoustic characteristic of the rolling bearing and rolling device, but also provide a source of concentrated stresses to lower the fatigue strength as well as deteriorate the toughness and corrosion resistance of the rolling bearing and rolling device. Therefore, it is not preferable that the large-size eutectic carbides exist in the material of the component parts of the rolling device.
Also, when the rolling bearing or other rolling device is used under a poor lubrication condition, for example, in an extreme case such as in the water, it is necessary to have good corrosion resistance. On the other hand, as for the life of the rolling bearing and rolling device, a wear resistance is especially also important.
When the inner and outer members and the rolling elements are respectively formed of ordinary material such as SUS440C or the like, if the rolling device is used under a severe condition such as in the water, then no oil film is produced between the rolling elements and the first contact surface which is the contact surface of the outer member with the rolling elements, or the second contact surface which is the contact surface of the inner member with the rolling elements. Accordingly, since the rolling elements are directly contacted with the inner and outer members, the damage form of the rolling device is represented by the lowered life or precision due to wear or corrosion but not the flaked strength. As an example, such heavy wear in the ball bearing can be reduced greatly by using ceramics such as silicon nitrides or the like as the material of the rolling elements. In this case, by using the ceramics as the material of only the rolling elements, not only an increase in the cost of the rolling bearing can be minimized but also the function of the rolling bearing can be improved outstandingly. When ordinary material such as SUS440C or the like is used for the races and ceramics is used for the rolling elements, the amount of wear of the rolling bearing can be reduced greatly to thereby be able to extend the life of the rolling bearing when compared with a case in which the races and rolling elements are all formed of SUS440 material, while the damage form of the rolling bearing shows a flaked damage involved with wear and corrosion.
However, since ceramics is little deformed elastically, the race in contact with ceramics receives a higher contact pressure when compared with a case in which stainless steel is used for the rolling elements, so that the race can be flaked and damaged starting from the large-size eutectic carbides contained therein. For this reason, use of ceramics cannot improve the life of the rolling bearing sufficiently. Also, the SUS440C material is insufficient in corrosion resistance, that is, when a rolling bearing formed of the SUS440C material is exposed under a corrosive environment such as in the water for a long time of period, the bearing corrodes and gathers rust starting from the Cr shortage layer in the periphery of the eutectic carbides, so that the precision of the bearing such as the roughness or the like is lowered to thereby reduce the life of the bearing. When the SUS440C material corrodes excessively, the rolling bearing cannot be used any longer.
On the other hand, in the case of the ball bearing that is built in a compact equipment such as an HDD, a VTR or the like, as the equipment becomes portable, a possibility that shocking loads are applied to the ball bearing is increased. In this case, since the ball bearing is small in size, even if the shocking load to be given to the ball bearing is relatively small, the races of the ball bearing can be deformed permanently to thereby deteriorate the acoustic characteristic of the equipment or generate variations in a rotation torque thereof, which results in the deteriorated performance of the equipment. Such permanent deformation occurs because the retained austenite contained in the steel forming the races shows a low yield stress.
The amount of the retained austenite can be reduced to almost 0% by tempering the races at a temperature of about 240.degree. C. when the races are formed of SUJ2, so that it is possible to improve the impact resistance of the ball bearing. However, as previously described, SUJ2 raises a problem that it has not sufficient corrosion resistance.
On the other hand, when the races of the ball bearing are formed of ordinary stainless steel such as SUS440C or the like having some corrosion resistance, even if the races are subzero treated after they are hardened, there is still left about 8-12 wt % of retained austenite and, further, this retained austenite is stabler than in the case of bearing steel, that is, it is little dissolved unless it is tempered at temperatures of 400-600.degree. C. Besides, although the retained austenite can be dissolved by tempering it at temperatures in the range of 400-600.degree. C., the hardness of the races is softened down to H.sub.R C 55-57 or less to thereby lower the rolling fatigue strength, wear resistance and the like of the races, which results in the shortened life of the rolling bearing including such races.
In addition, another problem arises. That is, in the above tempering process, Cr contained in the base material of the bearing is caused to separate from the present base material in the form of carbides. Not only the thus separated carbides are softened as the tempering temperature rises, but also the existence of such carbides lowers the corrosion resistance of the races to a great extent.
Unexamined Japanese Patent Publication No. Sho. 61-163244 discloses a rolling bearing of stainless steel in which formation of eutectic carbides is restricted by reducing the contents of C and Cr, so that the acoustic characteristic, fatigue strength and the like of the rolling bearing can be improved to a great extent. However, this disclosure does not teach any clear description as to the dimensional stability of the rolling bearing, the impact resistance of the rolling bearing caused by the amount of retained austenite, the wear resistance of the rolling bearing in the case where it is subjected to the tempering process at a high temperature, and the corrosion resistance of the rolling bearing. Various problems as described above in the rolling bearing may arise similarly in rolling device such as the linear guide and ball screw.